fix: accept day suffix in validityDuration as documented

[xrl]

validityDuration is documented as a human-readable duration (100d12h, with defaults described as 90d/365d; see api/v1alpha1/etcdcluster_types.go:126), but parseValidityDuration hands the string straight to time.ParseDuration, which has no d unit. Setting 365d makes every reconcile fail with failed to parse ValidityDuration: time: unknown unit "d" in duration "365d". The samples were switched to 8760h to sidestep it; the API doc still promises days.

This expands a leading day segment to hours before parsing. Plain Go durations and the empty-string default are unchanged. Table-driven tests cover day-suffix, mixed, plain, default and invalid inputs.

Rebased onto main after the #362 rewrite. make test and make lint pass.

[kubernetes-prow]

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[kubernetes-prow]

Hi @xrl. Thanks for your PR.

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